Explanation

(b)
Phosphorus pentoxide acts as a powerful dehydrating agent. It dehydrates HNO_{3} to N_{2}O_{5,} H_{2}SO_{4} to SO_{3}, HCIO_{4} to Cl_{2}O_{7} etc.
4 HNO_{3} + P_{4}O_{10} ⟶2N_{2}O_{5} + 4HPO_{3}
2H_{2}SO_{4} + P_{4}O_{10} ⟶2SO_{3} + 4HPO_{3}
4 HCIO_{4}+ P_{4}O_{10}⟶2Cl_{2}O_{7} + 4 HPO_{3}